This invention relates to a tone signal genration device performing removal of an aliasing noise and tone color control utilizing a digital filter operation and, more particularly, to a tone signal generation device capable of performing a digital filter operation of a good quality with a relatively simple hardware struccture.
In an electronic musical instrument, a digital filter is utilized for controlling the tone color of a digital tone signal or removing noise. For example, in a tone synthesis method called a pitch non-synchronizing type tone synthesis method according to which a tone signal is synthesized by conducting sampling with a constant sampling frequency irrespective of the frequency of a tone to be synthesized, the frequency of the tone and the sampling frequency are generarlly in non-integer ratio relation and, accordingly, as will be apparent from the sampling theorem, an aliasing noise which is not harmonic with the tone frequency tends to be generated and removal of such aliasing noise becomes necessary. For removing such aliasing noise contained in a pitch non-synchronizing type tone signal, it has been proposed to cause a tone signal to pass through a digital filter of characteristics capable of removing the aliasing noise (Japanese Preliminary Patent Publication No. 90514/1986 which corresponds to U.S. Pat. No. 4,701,956).
In such method according to which a tone signal is caused to pass though a digital filter for the removal of aliasing noise, a filter operation of high accuracy must be performed by using a sufficiently large number of filter orders. This however poses the problem that it necessitates a complicated filter construction. If, for example, the digital filter is one of 96 orders, filter operations for all of the 96 orders must be performed so that a circuit design enabling all of such filter operations must be employed. The same problem arises in not only a digital filter for removing aliasing noise but digital filters for tone color control and other purposes, i.e., in the prior art digital filters, filter operations must be performed for all orders if desired filter characteristics should be realized.